compressor-surge

name	compressor-surge
description	Compressor surge — surge line, Greitzer B parameter, rotating stall, anti-surge control, recycle valve, suction throttle, surge margin, API 670 vibration, active magnetic bearing damping.
metadata	{"priority":7,"promptSignals":{"phrases":["compressor surge","surge control","anti-surge valve","surge margin","rotating stall","Greitzer B parameter"],"minScore":3}}

Compressor Surge — Complete Skill

Surge Fundamentals

Surge: complete flow breakdown and reversal in compressor; system instability Onset: when compressor operates at flow rates below surge line on compressor map Mechanism: positive slope on pressure-flow curve → unstable equilibrium → limit cycle oscillations

Distinction from rotating stall:

• Rotating stall: one or more stall cells rotate around annulus; flow not reversed; local flow breakdown
• Surge: full annulus breakdown; global flow reversal; violent; damaging to machine

Surge indicators:

• Loud periodic bangs (0.5–2 Hz for industrial compressors; slower for large machines)
• Large pressure/flow oscillations; vibration
• Rotor axial thrust reversal → thrust bearing damage
• Temperature spikes at discharge
• Damage to seals, blades, diaphragms

Compressor Map and Surge Line

Surge line: connects surge points at different speeds on P-Q or head-flow map Surge margin (SM): SM = [(Q_design - Q_surge) / Q_design] × 100% [%] Or: SM = [(H_surge/H_design) - 1] × 100% (pressure-based definition; less common)

API 617 requirement: SM ≥ 10% between normal operating point and surge line at any speed

Operating line vs. surge line: Operating line: system resistance curve intersects compressor curve If system resistance increases (valve closes, pipe plugs) → operating point moves left → toward surge

Stonewall (choke): right side limit; sonic condition at some flow path throat

Greitzer B Parameter

Stability criterion for whether system enters rotating stall or surge: B = U/(2ω_H L_c) = (U/2) × √(V_p / (A_c L_c a²))

B = Greitzer parameter; U = rotor tip speed [m/s]; ω_H = Helmholtz frequency; L_c = effective plenum inlet duct length; V_p = plenum volume; A_c = duct area; a = sound speed

Critical B value: B < B_crit: rotating stall (stable oscillation) B > B_crit: full surge (unstable) B_crit ≈ 0.5–1.0 (depends on compressor characteristic slope)

Implication: large downstream plenum volume → high B → more prone to full surge Piping system design: minimize unnecessary plenum volume between compressor and downstream throttle

Anti-Surge System

Recycle (Anti-Surge) Valve

Function: when compressor approaches surge → open recycle → return gas from discharge to suction → increase compressor flow → move operating point away from surge

Valve type:

• Control valve: proportional control; modulates continuously
• Trip valve: fast-opening (full open in < 1 sec) for surge trip

Valve sizing: Q_recycle_min = Q_surge × 1.1 - Q_process [recycle must provide this additional flow when process demand drops] Cv = Q_recycle / √(ΔP_recycle / SG) [size for full bypass at maximum recycle condition]

Loop cooling: recycled gas may need cooling (gas heats with recycling); cooler in recycle loop

Surge Control Algorithm

Standard approach (line-based):

1. Calculate compressor "position" relative to surge line using measured P_suction, P_discharge, T_suction, flow Q
2. Define control line = surge line × 1.1 (10% margin)
3. If flow drops to control line → PID opens recycle valve
4. Rate of change algorithm: detect rapid approach → pre-emptive opening

Flow measurement correction: Corrected flow: Q_corr = Q × √(T_in_actual/T_in_ref) × (P_ref/P_in_actual)

Suction Throttle (Alternative)

For centrifugal with variable inlet guide vanes (IGV): Close IGV → reduce flow → maintain operating point with preswirl Reduces surge margin consumed → better part-load efficiency

IGV effect on surge line: Closing IGV shifts both surge line and performance curve → maintains relative margin Effective from 100% to ~50% capacity

Dynamic Surge Analysis

Simplified lumped-parameter model: ρ L dV/dt = P_c - P_s - ρ V² / (2 K_T) [V = flow velocity; P_c = compressor pressure; P_s = system; K_T = throttle constant] dP_plenum/dt = (ρ a² / V_plenum) × (Q_c - Q_process)

Limit cycle (steady surge): Amplitude determined by compressor curve shape and B parameter Frequency: ω_surge ≈ ω_H × √(1 - B² × dΨ/dΦ × slope)

Damping injection via AMB: Active Magnetic Bearings with surge detection controller → add artificial damping to rotor Shifts surge onset to lower flows → increased operating range by 5–15%

Rotating Stall

Stall cells: 1–N_blade stall cells rotate at 25–70% of rotor speed Detection: vibration at stall cell frequency = n × f_stall

Effect: Moderate: reduced performance; increased bearing loads; noise Severe: can progress to full surge; blade fatigue

Recovery: typically auto-recovers when back-pressure reduced; no hardware required Some compressors designed to operate in stall (fan laws on stalled side)

Vibration Monitoring (API 670)

Non-contacting proximity probes (XY pair): Mounted at each journal bearing; continuous orbit/vibration monitoring Alert level: 1× amplitude > 0.4 × (12000/N)^0.5 mils pp (typical) Trip: 2× alert (or 50.8 μm pp, whichever less)

Surge event signature: Large broadband vibration surge → thrust bearing axial motion > 0.5 mm → trip on axial position

Axial position monitor: API 670 requires axial probe; trip if axial displacement > ±0.75 mm from center (typical)

Standards

Standard	Scope
API 617	Centrifugal compressors for petroleum industry
API 670	Machinery protection systems; vibration limits
ISA-S67.04	Surge control systems
API 521	Pressure relief sizing

Output

Provide: surge margin SM [%] at operating point (vs. API 617 ≥ 10%), Greitzer B parameter, anti-surge valve Cv and type (control/trip), recycle flow Q_recycle [m³/hr] at surge condition, suction throttle (IGV) use, control algorithm (line-based/rate-of-change), recycle cooler requirement (temperature at recycle inlet [°C]), axial position trip limit [mm], vibration limits [μm p-p per API 670], and applicable standard (API 617, API 670).